Group-III nitride materials such as gallium nitride are wide bandgap semiconductors used in the production of electrical and opto-electronic devices such as blue light emitting diodes, lasers, ultraviolet photodetectors, and power transistors. There are currently no cost effective, lattice matched substrates on which these crystalline materials can be epitaxially grown. Common substrates for the growth of these materials include sapphire, silicon, gallium arsenide, and silicon carbide. Each of these materials has significant lattice mismatch with respect to the gallium nitride crystal structure. For example, the lattice mismatch for gallium nitride on sapphire is about 16%, gallium nitride on silicon carbide is about 3.1%, and gallium nitride on silicon is about 17%.
The lattice mismatch between the substrate and the epitaxial overgrown layer results in defects in the periodic crystal structure of the epitaxial layers. The most common defect is called a threading dislocation, which is essentially a misalignment in the lattice of the crystal structure. Dislocation densities above 104 cm−2 typically degrade performance of both optical and electronic devices made with such structures by carrier scattering, catalyzing impurity movement, roughening interfaces, and serving as a parasitic defect or recombination site. In order to preserve smooth interfaces and reduce dislocation densities, a variety of mitigation and density reduction approaches have been proposed in the past.
Techniques such as molecular beam epitaxy (MBE) or metal-organic chemical vapor deposition (MOCVD) for the growth of gallium nitride typically produce material with dislocation densities on sapphire or silicon carbide greater than 109 cm−2 and on silicon greater than 1010 cm−2, with very thick films having lower densities. High quality commercial templates can currently be obtained with dislocation densities that are about 5×108 cm−2 for 3 μm material and about 5×107 cm−2 for 10 μm material, the latter MOCVD films grown by epitaxial lateral overgrowth. In the most successful methods, threading dislocations are not replicated by employing a lateral growth mode and channels are created so that they can either terminate or turn around.
In the lateral overgrowth approach, the underlying substrate is patterned using a photomask and material is grown in windows opened to the substrate. As the crystal grows, the window tends to overgrow the masked area. In this overgrown area, the threading dislocation density is significantly lower than in the window area (up to 4 orders of magnitude lower). A disadvantage of this technique, however, is that the substrate must be patterned and, in turn, area is sacrificed. Other approaches to reduce dislocation densities in gallium nitride include insertion of low temperature layers, insertion of silicon nitride layers, and insertion of nanostructured layers of quantum dots. These structures provided dislocation densities of 107 cm−2 using either multiple layers or thick film growth. In the last case, the nanofeatures are thought to serve as end points for the threading dislocations. In all of these approaches to reduce dislocation densities, however, thick layers are needed, which causes undesirable stress in the layers, many processing steps are needed, and typically several growth reactors are required.